{WasmInstructionBuffer} was basically a wrapper around {AssemblerBuffer}
which remembered the last {AssemblerBuffer} on {Grow()}. Since the
{Assembler} itself already keeps track of the latest {AssemblerBuffer},
this functionality is mostly redundant. All we need instead is a method
to retrieve the {AssemblerBuffer} from the {Assembler}.

This CL thus removes {WasmInstructionBuffer} and instead adds
{AssemblerBase::ReleaseBuffer}.

R=jkummerow@chromium.org, mslekova@chromium.org
CC=dlehmann@google.com

Bug: v8:11714
Change-Id: Id07945b67992802a6177bf09e5f5c5be08f657b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2982013
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75336}